Current smoke detector technology is based on one of two general approaches. Photoelectric-based detectors are based on sensing light intensity that is scattered from smoke particles. Light from a source (LED) is scattered and sensed by a photosensor. When the sensor detects a certain level of light intensity, an alarm is triggered. Ionization-type smoke detectors are based on a radioactive material that ionizes some of the molecules in the surrounding gas environment. The current of the ions is measured. If smoke is present, then smoke particles neutralize the ions and the ion current is decreased, triggering an alarm.
Referring to FIG. 1, ionization sensor smoke alarms 100 contain a small amount of radioactive material, americium 101, embedded in a gold foil matrix within an ionization chamber 103. The matrix is made by rolling gold and americium oxide ingots together to form a foil approximately one micrometer thick. This thin gold-americium foil is then sandwiched between a thicker (˜0.25 millimeter) silver backing and a 2 micron thick palladium laminate. This is thick enough to completely retain the radioactive material, but thin enough to allow the alpha particles 102 to pass.
The ionization chamber 103 is basically two metal plates 104 a small distance apart. One of the plates 104 carries a positive charge, the other a negative charge. Between the two plates 104, air molecules received through the screen 105, made lip mostly of oxygen and nitrogen atoms, are ionized when electrons are kicked out of some molecules and picked up by other molecules as a result of collisions with alpha particles 102 from the radioactive material 101. The result is oxygen and nitrogen molecules that are both positively and negatively charged, such as NO+, O2−, OH−, HCO3+, and many other similar ions.
FIGS. 1 and 2 illustrate how ionization technology works. Referring to FIG. 1, the positive atoms flow toward the negative plate, as the electrons or negative ions flow toward the positive plate. The movement of the charges registers as a small but steady flow of current. Referring to FIG. 2, when smoke particles 106 enters the ionization chamber 103, the current is disrupted as the smoke particles 106 attach to the charged ions and restore them to a neutral electrical state. The large smoke particles 106 can shield the electric charge due to their size, or neutralize the charge through a chemical reaction. The net result is that fewer charged ions make it to the electrode. This reduces the flow of current between the two plates 104 in the ionization chamber 103. When the electric current drops below a certain threshold, an alarm is triggered.
There are problems with the radioactive material that is currently used as an ionizer.
1. The radioactive material is a small amount and does not pose a health hazard to the homeowner as long as the material is not tampered with. It is possible that someone could tamper with the americium-based smoke detector and inadvertently inhale or ingest the americium. This can be a serious health hazard.
2. Although the amount of americium in each smoke detector is small (about 1 microCurie), the accumulated amount of material can add up. The typical user disposes of the smoke detector by throwing it in the household trash. It is possible that this material can then find its way into recycled material that could then find its way back into a home in a form that is not so innocuous as the original smoke detector.